The present invention relates to an improved multigrip blind rivet, of the type which has circumferential grooves spaced at intervals along its length.
A known form of multigrip blind rivet has circumferential grooves spaced at intervals along its length. In the setting of this type of blind rivet, the grooves collapse in an axial direction and the rivet body portions between the grooves expand radially to form a characteristic "cottage loaf" setting.
For this type of rivet, there are two basic design considerations that require improvement:
a) In the formation of the grooves the rivet body material is work hardened in the groove portion and, unless there is close control of material displacement, there can be cracking of the material on setting, resulting in a very low tensile strength of the rivet and, in extreme conditions, premature failure of the joint. PA1 b) Due to variations in raw material composition and specification, and manufacturing tolerances, the setting and checking of the machinery for making the grooves require constant attention.
European Patent No 0 663 536 B1 describes and claims a blind rivet with a tensile mandrel comprising a mandrel head and a mandrel shank, and with an essentially cylindrical rivet sleeve which surrounds the tensile mandrel over part of its length, and which comprises at least one weakening zone in the longitudinal extent, wherein each weakening zone is formed by several recesses which are distributed in the circumferential direction and essentially arranged at the same axial position, characterised in that the bottoms of the recesses in each case essentially form a secant to the cylinder circumference of the rivet sleeve or run in a radial direction within this secant, so that at their centre they are made deeper in a direction towards the centre axis of the rivet sleeve than at the edges.
The rivet according to European Patent No 663 536 has the disadvantage that it is relatively expensive to manufacture, requiring close control of the process of manufacture. In particular, the indents which are opposed to each other have to be matched precisely.